Lipoprotein(a) particle number assay without error from apolipoprotein(a) size isoforms

Abstract Background Lipoprotein(a) [Lp(a)] is an important cardiovascular risk factor, but clinical immunoassays are flawed. Apolipoprotein(a) [apo(a)], the characteristic protein of Lp(a), contains a variable number of kringle repeats (size isoforms) that make accurate measurement of Lp(a) difficult. We developed a sandwich enzyme immunoassay that uses a murine monoclonal anti-apo(a) antibody for capture and a polyclonal anti-apolipoprotein B (apo B) for detection. Because Lp(a) contains one molecule each of apo(a) and apo B, the assay measures the number of Lp(a) particles [Lp(a)-P] in the circulation without bias due to apo(a) size isoforms. Methods After developing and choosing the best anti-apo(a) clone for Lp(a) capture, we identified suitable reagents and ELISA conditions, and validated assay performance (precision, linearity, limit of detection, interferences, and apo(a) size isoform bias). Results The Lp(a)-P assay was precise with within-run precision of 5.5% to 7.2% and total imprecision of 6.9% to 12.1%. The assay had a limit of detection of 13 nmol/l and was linear from 2 to 499 nmol/l. There was no interference from plasminogen or apolipoprotein B up to 80 and 200 mg/dl, respectively, and bias plot showed no bias related to apo(a) size (kringle 4 type 2 repeats). Conclusions Lp(a)-P assay is sensitive, precise and linear over a wide analytical range and is a suitable alternative for laboratories concerned about inaccuracy due to apo(a) size polymorphism and the poor performance of immunoturbidimetric assays.